Cryo-EM Sample Preparation: Best Practices
Complete guide to preparing cryo-EM grids, from protein purification to vitrification and screening.
Sample Requirements
Ideal cryo-EM samples are: monodisperse (single species), concentrated (0.5-5 mg/mL for single particle), in a buffer without glycerol or high salt, and freshly purified. Size exclusion chromatography (SEC) immediately before grid preparation is strongly recommended to remove aggregates.
Grid Selection
The most common grids are Quantifoil R1.2/1.3 and R2/2 on copper or gold 300-mesh supports. Gold grids reduce beam-induced motion but are more expensive. For difficult samples, consider UltrAuFoil, C-Flat, or graphene-oxide coated grids. Grid choice significantly impacts ice quality and particle distribution.
Vitrification: Vitrobot vs. Chameleon
The Vitrobot Mark IV is the standard plunge-freezing device, using filter paper blotting to control ice thickness. Typical settings: 100% humidity, 4°C, 3-5 second blot time, blot force 0-2. The Chameleon (SPT Labtech) uses piezo-dispensing onto self-wicking grids, enabling faster freezing and time-resolved experiments with smaller sample volumes.
Troubleshooting Common Problems
Thick ice: reduce blot time or increase blot force. Preferred orientation: add small amounts of detergent (0.01% DDM) or use tilted data collection. Empty holes: increase protein concentration or try different grid types. Aggregation: ensure sample is freshly purified and check for air-water interface effects.
Frequently Asked Questions
How much protein do I need for cryo-EM?
For a typical single-particle project, you need 3-5 µL at 0.5-5 mg/mL for initial screening (2-3 grids), plus 20-50 µL for optimizing conditions and collecting final datasets. Total: approximately 50-200 µg of purified protein.
What buffer should I use?
Common cryo-EM buffers: 20 mM HEPES or Tris pH 7-8, 150 mM NaCl, optional 1-5 mM reducing agent. Avoid: glycerol (>5%), sucrose, high PEG, high detergent concentrations. These can cause ice quality issues.
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